In a typical cold-cathode fluorescent lamp, a phosphor particle film is formed on an inner surface of a translucent glass bulb having electrodes arranged at both end portions thereof. In this glass bulb, a mixture of an ionizing gas including mercury and one or two or more kinds of rare gas/gasses are filled. When a positive column discharge starts between the electrodes, the mercury in the bulb is excited and ionized, and ultraviolet rays of 185 nm and 254 nm as resonance lines generated due to the mercury excitation are converted into visible light by phosphors on the inner surface of the bulb.
In a recent trend, the lamp current in a cold-cathode fluorescent lamp as a backlight source for a liquid crystal display has been increased due to decrease in tube diameter for providing a thinner liquid crystal display and also for raising the luminance of the liquid crystal display. The decrease in the tube diameter and the raised current will increase the rate of radiation of an ultraviolet ray having a wavelength of 185 nm. The increase of radiation rate of the resonance line at the short-wavelength side will increase a rate of deterioration of luminance of a fluorescent lamp over lighting time.
Factors that lower the luminance can be classified into three categories. A first factor is the coloring of glass. In most cases, this results from solarization due to the ultraviolet rays generated by a low-pressure vapor discharge of mercury and also due to collision of mercury ions. For suppressing the coloring of glass, it is proposed and practiced to form a base protective film made of Al2O3 fine particles or the like between a phosphor layer and a glass bulb in order to suppress irradiation of the glass bulb with ultraviolet rays.
However, degradation of the phosphor, which is a second factor of deterioration of luminance, cannot be suppressed only by covering the glass bulb surface with the base protective layer. Degradation of the phosphor is accelerated by irradiating with the above-described resonance line at the short-wavelength side (an ultraviolet ray having a wavelength of 185 nm). JP-07(1995)-316551 A proposes suppressing degradation of a phosphor by covering surfaces of the phosphor particles with a continuous coating layer. The reference discloses phosphor particles covered with a continuous coating layer by a sol-gel method using a solution of metalalkoxide. The phosphor particles are supplied onto the inner surface of the glass bulb after a coating of the particle surfaces. Ion impact to the phosphor can be eased by forming a phosphor layer in this manner.
However, the initial flux will be reduced remarkably when the entire phosphor surfaces are coated. Moreover, the intrusion of mercury into gaps among the phosphor particles cannot be suppressed by only forming a uniform coating film on each of the phosphor surfaces. A large amount of mercury exists in the glass bulb due to ambipolar diffusion. The ambipolar diffusion is a phenomenon in which mercury ions re-bind to electrons to be neutralized electrically. The mercury enters inside the phosphor layer and is physically adsorbed in the surfaces of the phosphor particles or the like, or they form compounds such as mercury oxide and amalgam and then are consumed.
Reduction of luminous efficiency due to the mercury consumption will result in a third factor to lower the luminance. It is known that mercury is consumed by forming amalgam with sodium. For suppressing consumption of the mercury, reduction of the sodium content in a glass bulb is proposed. However, the consumption of mercury cannot be suppressed even by adjusting the composition of the glass bulb. The consumption of mercury is accelerated when Al2O3 fine particles are blended in the phosphor layer to increase the film strength. Probably, this is caused by a large specific surface area of the Al2O3 fine particles.
Though measures for the respective factors that lower the luminance have been proposed as described above, these measures are not so sufficient when considering the above-described three factors comprehensively. The above-described measures can even degrade other properties such as the initial flux. Moreover, the conventional measures cannot improve the film strength while suppressing deterioration of the luminance.